That is not correct. "Free floated" on an AR just means that the foregrip does not contact the barrel. Some free floated fore ends do require special barrel nuts / attachments, however.
OP: Do you absolutely need a quad rail? To me they just add more unneeded weight. Forends that come with rail sections that you can bolt on to wherever you need to attach an accessory are becoming very common these days.
Troy Industries has rails like these that easily attach to a standard barrel nut for both rifles with a low profile gas black and those with a FSB in the $200 range. They also have quad rails if you absolutely need one.